express-rate-limit中间件每隔一定时间会清空统计的各个IP访问次数

看了下源码，通过设置var interval = setInterval(this.resetAll, windowMs)来进行每隔windowMs豪秒清空统计次数。 但随后又调用了interval.unref()来暂停该定时器。请问为什么要暂停该时时器呢？

	function MemoryStore(windowMs) {
	var hits = {};

	this.incr = function(key, cb) {
	if (hits[key]) {
			hits[key]++;
	} else {
		hits[key] = 1;
	}

  cb(null, hits[key]);
	};

	this.resetAll = function() {
		hits = {};
	};

	// export an API to allow hits from one or all IPs to be reset
	this.resetKey = function(key) {
		delete hits[key];
	};

	// simply reset ALL hits every windowMs
	var interval = setInterval(this.resetAll, windowMs);
	if (interval.unref) {
		interval.unref();
	}
	}

	module.exports = MemoryStore;